When electronic module power consumptions were lower, it was not necessary to make special provisions for removing heat from the circuit boards of the electronic modules to the box structure or mounting rack in which they were contained. As electronic module power consumption has increased, however, heatsinks and other thermally conductive structures were added to the module to conduct heat from the module to the walls of the mounting rack. It is now also common to have air cooling built into the side walls of the mounting rack. Electronic module power levels have now risen so high that liquid cooling or other more efficient heat transfer methods must be developed.
There are several approaches for continuing the use of air for cooling high power modules. One approach involves supplying cooling air at much lower temperatures and increasing the supply of the cooling air. This solution is expensive and can cause condensation of moisture on the air ducts if the air temperature is below the ambient dew point.
A second approach involves blowing cooling air directly over the surface of the electronic components on the module. This solution, however, is undesirable in some applications because of the chance that the cooling air might be carrying undesirable contaminants or moisture.
A third approach is to make the module heatsinks hollow and run the cooling air directly through the heatsink. Since conductive heat transfer resistances are minimized with this method, the cooling air does not have to be so cold.
Newer high-power density modules are being designed to operate within a safe temperature range by providing conductive heatsinks which conduct the heat to much colder liquid-cooled walls of the mounting rack. In retrofit situations, however, it is desired to replace older, low-power density modules with the newer, high-power density modules. In this retrofit situation, special provisions for cooling must be made. If cooling air is already available, the air must either flow through the module or the cooling system must be replaced to provide much cooler air or liquid flowing through the walls of the mounting rack. Further, a high power electronic module with a hollow core heatsink designed for flow-through air cooling may operate at excessive temperatures in a conductively cooled box if there is not enough conductive material in the flow-through heatsink to efficiently conduct the heat to the side walls of the module rack.